The present invention relates to laser texturing of magnetic data storage media and more particularly to processes for controlling the shapes and orientations of texturing features formed in such texturing processes, as well as the resulting storage media.
Laser texturing of magnetic disks, particularly over areas designated for contact with data transducing heads, is known to reduce friction and improve wear characteristics as compared to mechanically textured disks. Texturing involves focusing a laser beam onto a disk substrate surface at multiple locations, forming at each location a depression surrounded by a raised rim, as in U.S. Pat. No. 5,062,021 (Ranjan) and U.S. Pat. No. 5,108,781 (Ranjan). Alternatively, as shown in International Publication No. WO 97/07931 published Mar. 6, 1977 (Meyer), the laser beam also can be used to form a bump or nodule at each of the multiple locations. In some cases, a dome or bump is surrounded by a ring.
In any event, the individual topographical features are circular or axisymmetric, more particularly symmetrical about a vertical central axis in the case of features formed on a horizontal surface. While textures based on these features exhibit considerable improvement over mechanically formed textures, the ongoing quest for lower transducing head flying heights and shorter times for accessing data lead to more stringent tribological textures, the ongoing quest for lower transducing head flying heights and shorter times for accessing data lead to more stringent tribological requirements.
Meeting these requirements has been difficult in view of certain structural characteristics of the circular, axisymmetric rims and nodules. These features include a relatively small radius of curvature, a relatively large nodule height as compared to diameter, and large rim height compared to rim width. The resulting abrupt changes in surface elevation cause turbulence at the head/media interface. This results in undesirably high levels of acoustic energy at take-off and landing of the transducing head, i.e. at disk accelerations and decelerations. The closed, rim-like features have exhibited a tendency to collect debris, and their radial component can generate a drag force thought to contribute to dynamic friction.
Therefore, it is an object of the present invention to provide magnetic data storage media with transducer contact regions textured for improved wear and reduced dynamic friction.
Another object is to provide a process for using a laser to texture the surfaces of magnetic data storage media, with more control over the shape and orientation and texturing features formed by the laser to impart a desired roughness.
A further object is to provide a laser texturing process that affords more consistency in forming asymmetrical features such as rims and nodules on textured magnetic media.
Yet another object is to provide magnetic media in which the orientation of non-circular, non-axisymmetric topographical features is determined with reference to the direction of transducing head accelerations and decelerations, to increase the performance benefits derived from the asymmetries.
To achieve these and other objects, there is provided a process for surface texturing a magnetic data storage medium, including the following steps:
a. directing a coherent energy beam from a source thereof toward a magnetic data storage medium;
b. locating beam shaping optics between the source and the storage medium, thereby shaping the beam to provide a shaped beam segment along which sections taken through the beam perpendicular to beam propagation have a selected non-circular sectional shape; and
c. causing the coherent energy beam to impinge upon a selected surface of the storage medium at a plurality of locations thereon, altering the topography of the selected surface at each of the locations by forming a non-circular texturing feature, while orienting the features with major axes thereof substantially aligned in a predetermined direction.
The features formed according to this process are of the same general types as previously mentioned forms, including nodules, bumps and rims, having a smooth, rounded or rotund character that leads to reduced friction and increased wear. In a departure from prior forms, features formed by the present process are non-circular or non-axisymmetric in the sense of being substantially elongated in a direction parallel to the major plane of the data storage medium. Thus, the nodules or bumps are oblong or oval, and the rims are elliptical rather than circular. In general, each feature has a major or xe2x80x9clongxe2x80x9d axis in the direction of elongation and a minor or short axis that is perpendicular to the major axis. The aspect ratio, i.e. the major axis/minor axis ratio, can be in the range of about 1.5 to about 10, and more preferably is in the range of about 4-6.
The formation of elongated features provides several advantages. One of these, specific to rim-like features is a reduced tendency to entrap material. The elongated, elliptical rims exhibit a height gradient in the direction of the major axis, from a maximum height along medial regions intersected by the minor axis, to minimal heights at opposite end regions intersected by the major axis. At sufficient elongation, the rim height at these end regions becomes negligible, forming interruptions or gaps in the otherwise elliptical rim. These gaps allow any material collected within the rim to escape, e.g. during storage medium accelerations and decelerations.
More generally, all forms of the elongated texturing features exhibit more favorable profiles when elongated. In particular, each nodule, rim or other feature has a height profile that varies, from a maximum height gradient or steepest profile in the vertical plane containing the minor axis, to a minimum height gradient or most gradual profile in the plane containing the major axis. Other planes yield intermediate gradients and profiles, with the maximum or minor-axis profile most like that of a similarly sized but axisymmetric feature.
Thus, a maximum benefit is realized when the elongated topographical features are correctly oriented, i.e. with their major axes to the extent possible aligned with the direction of transducing head travel relative to the storage medium during accelerations and decelerations. In disk-shaped storage media, this direction is circumferential with respect to the disks. As a result of this orientation, the transducing head, when moving on or over the dedicated contact region, encounters more gradual changes in height. This reduces turbulence at the head/medium interface, reduces dynamic friction, reduces stiction, and permits lower head flying heights. Wear characteristics are improved, because the elongated features, when properly oriented, more effectively withstand contact with the transducing head.
The beam shaping optics can take a variety of forms, the most simple being a single wedged prism, with a pair of cylindrical lenses being more suitable. The most preferred version of beam shaping optics comprises a pair of complementary wedged prisms. The wedged prisms are preferred due to their plane surface refraction of the coherent energy beam, which preserves beam collimation and propagation factor M2 while minimizing aberrations and distortions. This enables a smaller focal spot size, suitable for forming smaller texturing features, while also enhancing control over the shape of the features.
Regardless of whether wedged prisms or cylindrical lenses are employed, using beam shaping optics to provide a specially shaped, non-circular (in section) laser beam affords effective control over the shape and orientation of elongated texturing features, and affords a high degree of flexibility in terms of selecting aspect ratios. This is in contrast to tilting the laser beam, a possibility mentioned in the foregoing Ranjan patents and Meyer international publication, for which an aspect ratio as low as 2 would require a severe tilt of 60 degrees from the preferred orientation perpendicular to the treated surface. The improved control and flexibility afforded by optically shaping the laser beam allow a customizing of texturing heretofore unavailable, not only in selecting the degree of elongation, but in the consistency with which features conform to the desired asymmetry.